Don't know anything about HDMI, but just wondering about the GTS 450, as mine will arrive tomorrow:

Does the 260.19 beta driver work will with the GTS 450? When searching for official Linux drivers on Nvidia's website it does not find anything, and Windows support only started with the 260.63 beta driver.

Thanks.

Read the link frenchy2k1 gave you. ALSA takes care of the audio portion.